The Basics of Compost Tea
Brewing Life: The Basics of Compost Tea (and Why Food Safety Matters)
In our last post, we kicked off our soil health series with an introduction to compost tea—a liquid extract of compost that’s become a go-to tool for many gardeners and regenerative growers. But before diving into recipes and techniques, it’s worth understanding what compost tea is, why it’s used, and what precautions should be taken—especially when growing food.
Check out the reading list. The best and most comprehensive book is “The Rodale Book of Composting”.
What Is Compost Tea?
At its core, compost tea is a liquid solution made by steeping high-quality compost in water, often with added oxygen (aeration) and microbial foods like kelp, or fish hydrolysate to boost microbial activity. The goal? To extract and multiply beneficial microorganisms—bacteria, fungi, protozoa, and nematodes—that support soil health and plant growth.
There are two main types:
Aerated Compost Tea (ACT): Brewed with constant oxygen using a pump, often for 12–48 hours, depending on temperature. This is my usual method.
Non-aerated Compost Tea (NCT): A simpler method where compost is soaked in water without added oxygen, though this method is less popular due to concerns about pathogen growth. This week I started a comfrey ferment, and a comfrey/nettle blend. Stay tuned for results.
Why Use Compost Tea?
Advocates of compost tea claim a range of benefits:
Microbial Inoculation: Introduces beneficial organisms that can improve soil structure and nutrient cycling.
Disease Suppression: Certain microbes may help outcompete or inhibit plant pathogens.
Foliar Feeding: When sprayed on leaves, compost tea may enhance plant immunity and deliver nutrients directly.
However, the science is still catching up. Some studies show measurable benefits, while others question the consistency and repeatability of results. As with many biological practices, context matters—soil conditions, compost quality, brewing method, and application technique all play a role.
Food Safety: A Serious Concern
While compost tea holds promise, it also carries potential food safety risks—especially when used on crops eaten raw, like lettuce or strawberries. Please check out this great resource for food safety. ksre.k-state.edu
Here’s why:
If made improperly, compost tea can harbor harmful pathogens such as E. coli or Salmonella, particularly when ingredients like manure-based compost or molasses are used in anaerobic or poorly monitored brews.
When sprayed on edible leaves, these pathogens can persist and pose a health risk if not properly managed.
To reduce risk:
Use only high-quality, fully matured compost—ideally plant-based or well-aged and tested.
Brew in clean, sanitized equipment and monitor oxygen levels if aerating.
Avoid using compost tea on edible parts of plants close to harvest, or only apply it to soil, not foliage.
Consider local regulations—some organic certification programs restrict or ban certain compost tea practices unless specific safety standards are met. In Kansas, we have specific resources from K-State University that provide some food safety regulation guidance. Go to their website for more information: ksre.k-state.edu
In Summary
Compost tea is an intriguing tool for building soil life and boosting plant health, but it’s not a silver bullet. Its success depends on quality ingredients, careful brewing, and mindful application—especially when growing food crops.
Whether you're a soil nerd or just compost-curious, stay with us as we continue steeping in the fascinating (and microbially rich) world of compost tea.